1. Field of the Invention
The present invention relates to a photoelectric-conversion apparatus and particularly relates to a metal oxide semiconductor (MOS) photoelectric-conversion apparatus.
2. Description of the Related Art
Photoelectric-conversion apparatuses have been rapidly growing in demand, as image-pickup devices of two-dimensional-image-input devices, notably digital-still cameras and video camcoders and/or one-dimensional-image readers, notably facsimiles and scanners. In the above-described photoelectric-conversion apparatuses, MOS-photoelectric-conversion apparatuses using a MOS transistor are used, so as to read a signal. In the MOS-photoelectric-conversion apparatus, both a photoelectric-conversion area and a peripheral-circuit area can be formed at the same time by performing complementary metal oxide semiconductor (CMOS)-manufacturing processing. Therefore, the MOS-photoelectric-conversion apparatus can be formed by performing a manufacturing processing that is easier than that performed for manufacturing a charge-coupled device (CCD).
When performing the CMOS-manufacturing processing, a contact configured to electrically connect a semiconductor area provided on a semiconductor substrate and/or a gate electrode of a transistor to upper wiring is used.
A shared contact is used for connecting the semiconductor area to the gate electrode by using a single conductor without concern for the two-dimensional distance between the semiconductor area and the gate electrode that are connected to the contact and/or the two-dimensional distance between contacts. The above-described technology is often used for a static random access memory (SRAM), since the technology is advantageous to form a smaller semiconductor device by doing layout.
Japanese Patent Laid-Open No. 2002-368203 discloses an example where the above-described shared contact is used for forming a CCD-photoelectric-conversion apparatus. According to Japanese Patent Laid-Open No. 2002-368203, the shared contact is included in an output-buffer circuit having a MOS transistor in which a gate-electrode unit is electrically connected to a floating-diffusion area, as a drive transistor. Subsequently, an electrical-charge-conversion coefficient is increased due to a reduced capacity.
According to Japanese Patent Laid-Open No. 2002-368203, the shared contact is provided in an output unit of a horizontal CCD, and provided in an area different from a photoelectric-conversion area where a photoelectric-conversion element such as a photodiode is formed. On the other hand, through investigations performed by inventors of the present invention, a new problem is found, the new problem being caused by providing the shared contact in the photoelectric-conversion area where the photoelectric-conversion element is provided.
The area of a contact hole used to form the shared contact should be larger than that of a contact hole used to form an ordinary contact. In that case, the etching rate of the contact hole used to form the shared contact becomes higher than that of the contact hole used to form the ordinary contact due to the microloading effect. Subsequently, the overetching amount of a contact-hole unit used to form the shared contact becomes larger than in the case where the ordinary contact hole is formed. When the overetching amount is increased in the photoelectric-conversion area where the photoelectric-conversion element is provided, an increased number of noises to the photoelectric-conversion element are produced and the image quality deteriorates.
Accordingly, the present invention has been achieved, so as to use a shared contact in a photoelectric-conversion area without increasing a noise to a photoelectric-conversion element.